Paraformaldehyde fixed tumor cells retained their immunogenicity to a degree comparable to that of X-irradiated cells under the optimal conditions. Cells fixed at lower temperatures below 10 degrees C showed greatly reduced tumor transplantation antigen (TTA) activity. This process of TTA inactivation at lower temperatures was fully restored by warming the cells above 10 degrees C before fixation. Fixation for longer than one hour or at concentrations of paraformaldehyde greater than 1% caused a decrease in immunogenicity. Fixed tumor cells could be stored for at least one month without loss of TTA activity.